Here’s a series of things to consider if your Aperture experience seems to suddenly slow down, then speed back up again later. Aperture is a resource hog, and really does benefit from having as much power available as you can possibly give it. The less your computer is handling other processes, the more it has for Aperture.

1. How much RAM do you have in your system?
   • Install a utility like FreeMemory to monitor your free memory (it puts a number in your menubar showing how much free RAM your system has at any given moment). When Aperture slows down, look at that number. Is it below 1GB or even approaching zero? RAM is cheap… get some more. If you can’t (or before you do), look at the rest of these questions to see if you can find a way to free up some memory.
2. How much free space is there on your system hard drive?
   • The Mac OS needs free disk space to write to when it runs out of RAM. And no, no amount of RAM you install will ever keep this from happening; it’s just a core part of the operating system. General wisdom seems to be that 15% of your hard drive should be free at any given time; this means if you have a 1TB drive, you should have at least 150GB free space. 
3. Are you running Time Machine?
   • Time Machine is the Mac’s built-in backup system, and it’s great. However if it kicks in while you’re running Aperture, it can slow things down. Actually it can slow down everything, but it seems more noticeable in Aperture. You can always stop a backup in progress from the Time Machine menu item, however that can take several minutes. Personally, I keep my Time Machine volume unmounted during the day and only turn it on once a week or so (but I also use a cloud backup… see next question). I used to use a utility called TimeMachineEditor that allows you to schedule Time Machine backups (normally they go every hour, and there’s no built-in way to modify this). If you do delay backups however, just keep in mind that, well, you’re delaying backups. Are you prepared to lose the last hour/day/week of work in the event of sudden total drive failure?
4. Do you have a cloud backup running, such as Backblaze or Carbonite?
   • I’m a huge fan of Backblaze, which runs constant online backup to the cloud. However, it can also slow down your system while running if it has a lot of changes to process. You can schedule this to only go at night, or like Time Machine, you can pause it at any time. If you see your available memory suddenly drop (by watching FreeMemory, as mentioned above), check the Backblaze menu in the menubar. If you option-click on it, you can pause an active backup (it will resume again automatically in an hour). Memory will often free back up within a minute or so, and you’ll have your speed back. You can also schedule this to only go once per day, if that works out better for you.
5. Do you have Dropbox installed?
   • Dropbox is a very popular service for keeping folders of stuff synced across multiple computers. But like your backups, it has to run in the background to copy files to and from the server. If there’s a lot of files (especially lots of little files) suddenly added to your Dropbox, you could see system lag as it updates. If you click on the Dropbox menu in the menubar, you’ll see the option to Pause Syncing. Just don’t forget to turn it back on again later. NOTE: Do not try to be clever and store your Aperture library in a Dropbox folder. Bad Things™ can happen.
6. Do you have any anti-virus, system monitors, or anything else you can think of that’s installed that runs in the background on a regular (or irregular) schedule?
   • Think about the rest of your system; are there other activities running in the background? If so, can any of them be scheduled to run at night when you’re sleeping? Or maybe something you installed that one time at band camp and no longer need? (if you don’t get the reference, don’t ask…)